JP2001206775A - Method for producing porous silicon nitride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a silicon nitride porous body suitable for separating gases. SOLUTION: The method of producing a silicon nitride porous body substantially comprising silicon nitride comprises heat treating a formed body containing silicon dioxide particles, carbon particles and metallic silicon particles having an average diameter of <=5 μm, wherein the total amount of the silicon dioxide particles, the carbon particles and the metallic silicon particles is >=90 mass %, in gaseous nitrogen.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a porous silicon nitride material suitable for gas separation.

[0002]

2. Description of the Related Art Silicon nitride has excellent properties such as heat resistance, corrosion resistance and strength, and is expected to be used as a gas separation filter and its base material in a high-temperature atmosphere or a corrosive atmosphere. Several methods for producing such a silicon nitride porous body have been proposed.

[0003] Japanese Patent Application Laid-Open No. Hei 6-256069 proposes a method of firing a molded body composed of silicon nitride particles, clay and oxide. Also, JP-A-7-187845,
JP-A-8-59364 and JP-A-6-24859 each disclose a mixture of silicon nitride particles and an organosilicon compound,
A method has been proposed in which a mixture of silicon nitride particles and polysilazane and a mixture of silicon nitride particles and a synthetic resin foam are used as starting materials, and a molded body is similarly fired. However, the method using these silicon nitride particles as a starting material has a problem that the cost is high because the silicon nitride particles are relatively expensive.

On the other hand, as a method of using metal silicon particles, Japanese Patent Application Laid-Open No. 1-188479 discloses a method in which a mixed powder of metal silicon particles and silicon nitride particles is used as a starting material and the nitriding ratio of the metal silicon particles is 50% or less. A method for obtaining a porous body has been proposed. However, in this method, since the nitridation ratio of the metal silicon particles is 50% or less, many metal silicon particles remain in the silicon nitride sintered body without being nitrided, which impairs the excellent heat resistance and corrosion resistance of silicon nitride. There's a problem.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a porous silicon nitride material which uses metal silicon particles as a starting material and has a high nitriding rate and is suitable for gas separation.

[0006]

The present invention comprises a silicon dioxide particle, a carbon particle and a metal silicon particle having an average particle diameter of 5 μm or less, and a total amount of the silicon dioxide particle, the carbon particle and the metal silicon particle. Is a heat treatment in a nitrogen atmosphere of a molded body having a content of 90% by mass or more to produce a porous body substantially composed of silicon nitride.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the method for producing a porous silicon nitride material of the present invention (hereinafter referred to as the present production method), silicon dioxide particles,
A molded article containing carbon particles and metal silicon particles having an average particle diameter of 5 μm or less and having a total content of the silicon dioxide particles, the carbon particles and the metal silicon particles of 90% by mass or more is used.

When the average particle diameter of the silicon dioxide particles and the carbon particles is less than 0.1 μm, the particles are liable to agglomerate and difficult to handle. On the other hand, if the average particle diameter exceeds 10 μm, some of the pores formed are undesirably too large for the gas separation substrate.
Therefore, the average particle diameter of the silicon dioxide particles and the carbon particles is preferably from 0.1 to 10 μm. Also,
The purity of the silicon dioxide particles and the purity of the carbon particles are not particularly limited, and are appropriately selected depending on the application. It is considered that the silicon dioxide particles and the carbon particles act as a pore-forming agent during the production of the porous silicon nitride.

Preferably, the molar ratio of Si in the silicon dioxide / C in the carbon is 0.4 to 0.67. If the molar ratio is less than 0.4, silicon dioxide remains in the sintered body, and the corrosion resistance of the obtained porous body is unfavorably deteriorated. If the molar ratio exceeds 0.67, carbon remains in the sintered body, and the carbon and the metal silicon particles partially react to form silicon carbide particles, which is not preferable.

The content of the silicon dioxide particles and the carbon particles is preferably 1 to 20 parts by mass based on 100 parts by mass of the compact. Less than 1 part by mass is not preferred because the porosity of the porous body is too small,
On the other hand, if it exceeds 20 parts by mass, the porosity of the porous body becomes too large and the mechanical strength is not sufficient, which is not preferable.

The metal silicon particles used in the present production method have an average particle diameter of 5 μm or less. When the average particle diameter of the metal silicon particles exceeds 5 μm, metal silicon particles that are not nitrided by the heat treatment in the subsequent step remain, and the average pore diameter of the porous body becomes too large. The purity of the metal silicon particles is appropriately selected depending on the purpose and application.

In the present production method, the total amount of silicon dioxide particles, carbon particles and metal silicon particles is at least 90% by mass in the molded product. If the total amount of silicon dioxide particles, carbon particles and metal silicon particles is less than 90% by mass in the molded product, a filter having desired characteristics cannot be obtained.

In the present production method, as a method for producing a molded body containing silicon dioxide particles, carbon particles and metal silicon particles, a usual ceramic molding method such as press molding, extrusion molding, or casting is appropriately employed. . At the time of molding, an organic binder may be added. Examples of such an organic binder include polyvinyl alcohol or a modified product thereof, starch or a modified product thereof, carboxymethylcellulose, hydroxylmethylcellulose, polyvinylpyrrolidone, an acrylic resin or an acrylic copolymer, a vinyl acetate resin or a vinyl acetate copolymer, And other organic substances can be used. The amount of such an organic binder to be added is 0.1 to 10 on the basis of 100 parts by mass of the molded body.
It is preferable to use parts by mass. Note that the pore-forming agent may also function as a binder for the molded body.

The heat treatment of the molded body is preferably performed at a temperature of 1100 ° C. to 1400 ° C. in a nitrogen atmosphere because the nitriding ratio becomes high. Temperature range 1100 ° C
If it is less than 3, nitriding of the metal silicon particles does not occur, or even if a reaction occurs, the reaction rate is extremely slow, which is not preferable. On the other hand, if the temperature range exceeds 1400 ° C., the metal silicon particles melt around the melting point of metal silicon (1410 ° C.), which is not preferable. If the temperature holding time is less than 5 hours, the nitridation of the metal silicon particles is insufficient, which is not preferable. If the temperature holding time is more than 24 hours, the nitridation rate hardly changes and the operating cost increases, which is not preferable. Therefore, the temperature holding time is preferably 5 to 24 hours.

The rate of temperature rise during the heat treatment depends on the size of the compact,
It is appropriately selected depending on the shape and the like, but a temperature of 60 to 600 ° C./h is preferable in terms of nitriding and sintering. Heating rate is 60 ° C /
When the length is less than h, the time required for forming the porous body is undesirably long. On the other hand, if the heating rate exceeds 600 ° C./h, a gas is rapidly generated due to the reaction between the silicon dioxide particles and the carbon particles during the heat treatment, and the porous body is damaged, which is not preferable. If the temperature is in the range of 1100 to 1400 ° C. even during the heating process, the elapsed time is added to the holding time.

The porosity of the porous silicon nitride obtained by the present production method is preferably 20 to 40%. The porosity is measured by the Archimedes method. If the porosity is less than 20%, the pressure loss increases, which is not preferable as a porous body. Further, if the porosity exceeds 40%, the strength is not sufficient, which is not preferable.

The average pore diameter of the porous silicon nitride obtained by this production method measured by a mercury intrusion method is 30 to 100 n.
m is preferable. When the average pore diameter is less than 30 nm, it can be a gas separation substrate for forming a gas separation membrane on the surface, but as a substrate, the pore size is too small and the porosity is increased. Also has very low gas permeation. On the other hand, if the average pore diameter exceeds 100 nm, it is difficult to form a gas separation membrane uniformly on the substrate surface without any defect, which is not preferable.

The amount of silicon contained as silicon nitride (hereinafter referred to as a nitriding ratio) based on the total silicon including the metallic silicon and silicon dioxide of the porous silicon nitride obtained by the present production method is preferably 90% or more. If the content is less than 90%, the properties of the silicon nitride porous body, such as heat resistance and corrosion resistance, are undesirably reduced due to the remaining metal silicon particles.

In this specification, the nitridation rate of silicon nitride is calculated from a change in mass. That is, as shown in Formula 1, in the reaction for producing silicon nitride, 3 mol of metal silicon reacts with 2 mol of nitrogen to form 1 mol of silicon nitride.

From equation 1, if all of the metallic silicon is silicon nitride, its mass will be 1.67 times ((3 × Si + 4
× N) / (3 × Si) = (3 × 28 + 4 × 14) / (3
× 28) = 1.67). That is, assuming that all of the metallic silicon is silicon nitride, the rate of increase in mass is 0.67. Here, the rate of increase in mass is a value represented by (x-1) / 1 = (x-1) when a substance having a mass of 1 g becomes xg.

The reaction for producing silicon nitride by nitriding a mixture of silicon dioxide and carbon is given by equation 2. From equation 2, the mass change of the mixture of silicon dioxide and carbon is (3 × Si
+ 4 × N) / (3 × Si + 6 × O + 6 × C) = 140 /
252 = 0.56. That is, the mixture of silicon dioxide and carbon having a molar ratio according to Formula 2 (hereinafter, referred to as a mixture of Formula 2) has a mass increase rate of −0.44 when silicon dioxide is entirely silicon nitride.

[0022]

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Here, assuming that the mass of the metal silicon is W ₁ and the mass of the mixture of the formula 2 is W ₂ , the mass fraction m of the metal silicon is
It is given by Equation 3. Assuming that the mixture of the metallic silicon having the relationship of the formula 3 and the formula 2 is completely nitrided, the mass increase rate Δw _ideal is given by the formula 4.

[0024]

(Equation 1)

The nitriding rate is obtained by calculating the actual mass increase rate Δw by Δw
_Divide by _ideal . For example, W ₁ is 900 g, W ₂ is m = 0.9 When 100 g. In this case, the mass increase rate Δw _ideal when completely nitrided is calculated by Equation 4.
Is 0.56. If the actual mass increase rate Δw is 0.4
In this case, the nitriding ratio is 0.4 / 0.56,
Calculated as%.

[0026]

Examples of the present invention (Examples 1 to 3, Examples 5 to 5)
Example 9) and Comparative Examples (Examples 4 and 10) are shown. [Example 1] A mixed powder of silicon dioxide particles and carbon particles having a molar ratio of Si / carbon in silicon dioxide of 0.56 to an average particle diameter (laser diffraction type particle size distribution analyzer (manufactured by Nikkiso Co., Ltd. Product name: Microtrac HRA)
The same applies hereinafter) in an amount of 5 parts by mass with respect to 100 parts by mass of metal silicon particles having a particle size of 3 μm. The average particle diameter of the silicon dioxide particles was 0.5 μm, and the average particle diameter of the carbon particles was 1 μm.

The starting materials were mixed and pulverized for 20 hours by a ball mill using ethyl alcohol as a dispersion medium. After drying, the average particle diameter of the crushed mixed powder was measured to be 2.0 μm.

This mixed powder was filled in a 50 mm × 50 mm press die, uniaxially pressed at a molding pressure of 19.6 MPa, and further subjected to cold isostatic pressure (CIP) molding at a molding pressure of 98 MPa to obtain a 50 mm × 50 mm × A 5 mm compact sample was obtained. The molded body sample was heated to 1300 ° C. at a rate of 6.7 ° C./min in a nitrogen atmosphere furnace.
Heat treatment for nitriding and sintering was performed in a nitrogen atmosphere for 2 hours.

The porosity of the obtained porous body (by the Archimedes method, the same applies hereinafter) was 35%. As for the pore characteristics, the average pore diameter was 70 nm, and 90% of the pore volume was 100 nm or less. In each example, the pore characteristics were measured with a mercury porosimeter (trade name: AutoSCAN-33, manufactured by Yuasa Ionics). The nitriding ratio calculated from the change in mass was 92%.

[Example 2] A mixed powder of silicon dioxide particles and carbon particles having a molar ratio of Si in silicon dioxide / C in carbon of 0.63 was prepared by mixing metal silicon particles 1 having an average particle diameter of 3 μm.
10 parts by mass was added to 00 parts by mass to obtain a starting material.
The average particle diameter of the silicon dioxide particles was 1 μm, and the average particle diameter of the carbon particles was 1 μm.

10 parts by mass of methylcellulose and 30 parts by mass of ion-exchanged water were added to 100 parts by mass of the starting material, and the mixture was kneaded for 30 minutes in a pressure kneader to prepare a forming clay. The obtained forming clay was extruded by an extruder to obtain a flat plate sample having a width of 120 mm, a thickness of 6 mm, and a length of 30 cm. After the plate sample was sufficiently dried with a dryer, the sample was heated at 1380 ° C. × 5 ° C./min in a nitrogen atmosphere furnace.
A nitriding treatment was performed for 12 hours.

The characteristics of the obtained porous body are as follows:
%, The average pore diameter was 100 nm, 90% of the pore volume was 150 nm or less, and the nitriding ratio calculated from the mass change was 90%.

EXAMPLE 3 100 parts by mass of metal silicon particles having an average particle diameter of 1.5 μm were mixed with a mixture of silicon dioxide particles and carbon particles having a molar ratio of Si in silicon dioxide / C in carbon of 0.5. 5 parts by mass with respect to the starting material. The average particle diameter of the silicon dioxide particles was 0.5 μm, and the average particle diameter of the carbon particles was 0.5 μm.

The starting materials were mixed and pulverized for 20 hours by a ball mill using ethyl alcohol as a dispersion medium. After drying, the average particle diameter of the pulverized mixed powder was measured to be 1.4 μm.

This mixed powder was filled in a 50 mm × 50 mm press die, uniaxially pressed at a molding pressure of 19.6 MPa, and further subjected to cold isostatic pressure (CIP) molding at a molding pressure of 98 MPa to obtain a 50 mm × 50 mm. A molded product sample of × 5 mm was obtained. The molded body sample in a nitrogen atmosphere furnace,
The nitriding treatment was performed at a temperature rising rate of 5 ° C./min at 1350 ° C. for 8 hours in a nitrogen atmosphere.

The characteristics of the obtained porous body are as follows:
%, The average pore diameter was 50 nm, 90% of the pore volume was 100 nm or less, and the nitriding ratio calculated from the mass change was 95%.

Example 4 In Example 1, the average particle diameter of the metal silicon particles was changed from 3 μm to 20 μm. The average particle diameter of the mixed powder of the silicon dioxide particles and the carbon particles after pulverization was 5 μm. Otherwise in the same manner as in Example 1, a porous body was obtained. The properties of the obtained porous body were as follows: porosity was 36%, average pore diameter was 100 nm, and pore volume was 9%.
0% had a pore diameter of 150 nm or less, and the nitriding ratio calculated from the change in mass was 90%.

Example 5 The procedure of Example 1 was repeated except that the amount of the mixed powder of silicon dioxide particles and carbon particles was changed from 5 parts by mass to 20 parts by mass. The properties of the obtained porous body were such that the porosity was 40% and the average pore diameter was 100 n.
In m, 90% of the pore volume was 200 nm or less in pore diameter, and the nitriding ratio calculated from the change in mass was 92%.

[Example 6] In Example 1, the heat treatment temperature was set to 13
Same as Example 1 except that the temperature was changed from 00 ° C to 900 ° C. The properties of the obtained porous body were such that the porosity was 43%, the average pore diameter was 500 nm, 90% of the pore volume was 1 μm or less, and the nitriding rate calculated from the mass change was 50%.
%Met.

[Example 7] In Example 1, the heat treatment temperature was set to 13
Same as Example 1 except that the temperature was changed from 00 ° C to 1500 ° C. The properties of the obtained porous body were such that the porosity was 32%, the average pore diameter was 500 nm, 90% of the pore volume was 1 μm or less, and the nitriding rate calculated from the mass change was 9%.
8%.

Example 8 In Example 1, the heat treatment time was set to 12
Same as Example 1 except that the time was changed from 1 hour to 1 hour. The properties of the obtained porous body were such that the porosity was 38%, the average pore diameter was 200 nm, and 90% of the pore volume was 5%.
00 nm or less, and the nitridation rate calculated from the mass change is 7
It was 0%.

Example 9 The procedure of Example 1 was repeated except that the molar ratio of Si in silicon dioxide / C in carbon was changed from 0.56 to 0.3. The properties of the obtained porous body were such that the porosity was 40%, the average pore diameter was 100 nm,
90% of the pore volume had a pore diameter of 200 nm or less, and the nitriding ratio calculated from the change in mass was 88%.

Example 10 The procedure of Example 1 was repeated except that the silicon dioxide particles and the carbon particles were omitted.
The properties of the obtained porous body are such that the porosity is 19%, the average pore diameter is 30 nm, and 90% of the pore volume is 50 nm in pore diameter.
It was as follows, and the nitriding ratio calculated from the mass change was 95%.

[0044]

According to the present production method, a silicon nitride porous body which is inexpensive and has a high nitriding rate and is suitable for gas separation can be easily produced. The porous silicon nitride obtained by the present invention is particularly suitable for a gas separation filter substrate having a pore diameter and a porosity.

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Claims (5)

[Claims] 1. A molding comprising silicon dioxide particles, carbon particles and metal silicon particles having an average particle diameter of 5 μm or less, and wherein the total amount of said silicon dioxide particles, said carbon particles and said metal silicon particles is 90% by mass or more. A method for producing a silicon nitride porous body, wherein the body is heat-treated in a nitrogen atmosphere to obtain a porous body substantially composed of silicon nitride. 2. The method according to claim 1, wherein the molar ratio of Si in the silicon dioxide / C in the carbon is 0.4 to 0.67. 3. The method for producing a silicon nitride porous body according to claim 1, wherein a total amount of the silicon dioxide particles and the carbon particles is 1 to 20% by mass in the molded body. 4. The heat treatment conditions are as follows:
4. The method for producing a silicon nitride porous body according to claim 1, wherein the heat treatment is performed in a nitrogen atmosphere at a temperature of from 1 to 1400 [deg.] C. 5. The porosity of the silicon nitride porous body is 20-4.
5. The method for producing a silicon nitride porous body according to claim 1, wherein the average pore diameter is 0% and the average pore diameter is 30 to 100 nm.